A direct modulation system of modulating an optical signal output from a light emitting element (for example, a laser diode) by increasing or decreasing a current flowing to the light emitting element has been used in optical communication systems. Japanese Unexamined Patent Publication No. S60-187075 describes a driver circuit that includes a field effect transistor (FET) to which an input signal is input and a laser diode connected in series to a drain of the FET. A series circuit including a resistor and a capacitor is connected in parallel with the laser diode. Japanese Unexamined Patent Publication No. H2-65188 describes a driver circuit that includes a source-grounded FET and a laser diode connected to a drain of the FET via a resistor. A series circuit including a variable capacitance diode and a capacitor is connected in parallel with the resistor. U.S. Patent Application Publication No. 2006/0159462 describes a driver circuit of a laser diode which generates a driving signal having emphasized transitions. The transitions are emphasized by adding a positive pulse to an input pulse at a rising transition thereof and subtracting a negative pulse from the input pulse at a falling transition. The positive pulse and the negative pulse have amplitude reduced from that of the input pulse. A reference voltage of the positive pulse and the negative pulse corresponds to an average voltage of the input pulse. The positive and negative pulses are generated by calculating an exclusive OR of the input signal and a delayed signal delayed from the input signal. Japanese Unexamined Patent Publication No. 2016-96221 describes a shunt driver circuit connected in parallel with a laser diode to which a bias current is supplied. The shunt driver circuit causes a shunt current bypassed from the bias current so as to increase or decrease a laser current.
In the case of driving the laser diode by the direct modulation system, relaxation oscillation appears depending on a transition between oscillation states of light emission and extinction of the laser diode. The relaxation oscillation causes overshoot and undershoot in an optical output waveform. Such waveform disturbances degrade an eye opening of an optical signal, and thus, need to be suppressed. Japanese Unexamined Patent Publication No. H4-290278, Japanese Unexamined Patent Publication No. 2007-324463, and Japanese Unexamined Patent Publication No. 2013-239641 describe a driver circuit loaded with an RC series circuit including a resistive element and a capacitor or an RLC series circuit further including an inductor in order to suppress a disturbance of a waveform.
In the case of electrically connecting a driver circuit formed on a semiconductor chip to a semiconductor laser element by wire bonding, an inductance of a bonding wire prevents transition speed of a drive current from being enhanced. The inductance can be reduced by shortening a length of the bonding wire. However, heat is more easily transmitted through a more shortened bonding wire, and thus, cooling efficiency is degraded in the case of controlling temperature of the semiconductor laser element with a thermo-electric cooler (TEC) or the like. Therefore, a driver circuit hardly affected by the inductance is required in order to improve high speed performance of an optical communication system.